DE VERNEUIL AND d'aRCHIAC ON RUSSIAN PALEOZOIC FOSSILS. 103 



PentameruS) which occupied an important place among the palaeo- 

 zoic fossils of Russia, but the species, with the exception of a small 

 variety of P. yaleatus, are all Silurian. Most of the species belong 

 to the Ural, but one, P. horealis (the representative of P. oblongus 

 of Sweden, Germany and America), is found in the provinces bor- 

 dering on the Baltic, and, like its analogue, indicates the limit be- 

 tween the lower and upper stages of the great Silurian deposit. The 

 Ural species are almost all peculiar to that locality ; three of them 

 (of which two resemble more or less closely P. Knightii) are, to- 

 gether with this latter species, characteristic of the Upper Silurian 

 rocks, and are widely spread, some of them in the northern and the 

 rest in the southern part of the chain ; and there is a fourth species, 

 P. galeatusy which is less common, but which appears both in the 

 Silurian and Devonian series as in Western Europe. 



With regard to Spirifer, there are no less than thirty-six species, 

 nearly a fourth part of the whole number known. If to these we 

 add twelve others cited by other authors, and four which are doubt- 

 ful, the number rises to fifty-one. 



The species S, glaber and S. Hneatus are the only ones which we 

 have found in Russia passing from one system to another, and if 

 others should be hereafter met with, we may be pretty confident 

 beforehand that their number will be small. It is only toward the 

 close of the Devonian system that this genus begins to be greatly 

 developed, and the lower rocks are only represented in Russia by 

 some species more or less anomalous, such as S. insularis, which is 

 common to Norway and Russia, and also by the group oi Equirostrata 

 and the Biforata. S. lynx^ which belongs to the latter group, is one 

 of the most remarkable. It is characteristic of the lower stage of the 

 Silurian system on this side of the Atlantic as well as on the other, 

 where it is widely spread through the states of Ohio, Kentucky, 

 Tennessee, and on the banks of the St. Lawrence. The Devonian 

 species are characterized, as in the rest of Europe, by their fine and 

 non-dichotomous plications (e. g. S, Murchisonianus, S. Anossofi^ 

 S.disjunctus, S. Verneuili, S. Glinkanus, S.Archiaci and S.tentacU' 

 lum), and some of them extend as far as the Altai Mountains, where 

 M. Tchihatchelf has found S. Vemeuili, and others more or less 

 analogous. The smooth species are more rare. The carboniferous 

 system affords twelve more species, but of most of them the develop- 

 ment is local, only three species, S, Mosquensis, S. incrassatus and 

 *S'. glaber, being found in several places and abundantly ; and of 

 these three the first-named is one of the most common and most 

 characteristic shells of Russia, and occurs also in Belgium and at 

 Cabrales in the Asturias. Following the law of progressive decrease, 

 the Spirifers are thinly scattered through the Permian rocks, where 

 they lire only represented by four, species confined to a small num- 

 ber of localities. 



The species of Orthis also observe a law of progressive decrease 

 in their vertical distribution, but, unlike the Spirifer, this com- 

 mences from the lower members of the Silurian system, wliere they 

 have already obtained their maximum of development. The neigh- 



